Accurate motion control is necessary in many applications involving electric motors. Typically, the control is needed over one or more of the motion parameters, i.e., speed, location and phase, with varying degrees of precision. Such control is generally achieved by automatic control systems, for example, phase-locked speed controllers.
FIG. 1 is a schematic illustration of a phase-locked speed controller system 18, as is known in the art. A rotation detector 22 detects the rotational speed .OMEGA..sub.E of a motor 20 and generates an output pulse train having a frequency f.sub.E and a phase .PHI..sub.E, in response thereto. External speed and phase reference values and a clock signal of a frequency f.sub.clock, generated by a clock generator 28, are input to a programmable reference 25, which generates a reference pulse train of a frequency f.sub.R and a phase .PHI..sub.R. A correction device 24 receives the feedback and reference pulse trains and generates a feedback pulse train of a frequency fE and a phase .PHI..sub.F. A phase-frequency comparator 30, comprising, for example, an edge-controlled digital memory network, as is known in the art, compares the frequency and phase of the feedback pulse train to those of the reference pulse train, and generates a duty cycle-modulated signal of having a duty cycle .gamma..sub.1 in response to the difference between the two sets of values. The duty cycle signal is input to an integrator 32 to further generate a low-frequency pulse width-modulated (PWM) signal. This signal is output to a motor drive 36, which drives motor 20 in response thereto.
Control systems like system 18 typically exhibit large and irregular overshoot, i.e., slow convergence of speed transients. These problems are mainly due to low-frequency reference and feedback signals, whose frequencies can be varied only in discrete steps, as well as low-frequency PWM driving signals and slow response time of the phase control system. The system is typically subject to phase errors due to a "dead zone" effect in the integration circuit at low duty cycle pulse widths. There is no accurate and reliable way of controlling motor speeds in the range below a few tens of rpm, employing the above-described control systems. Similarly, there is no straightforward way for systems like system 18 to control with any accuracy the shaft position of the motor.
Attempts at improving one or more of the controlling parameters of phase-locked control systems have been performed mostly in relation to specific, dedicated applications, such as in video and digital audio recorders and players, where rapid response, i.e., rapid phase convergence during speed transients, is mandatory.
For example, U.S. Pat. No. 4,795,950, to Ota, et al., which is incorporated herein by reference, describes methods and apparatus for phase control of video and audio digital recorder motors with rapid phase convergence using a resettable phase signal generating circuit for producing a reference phase signal. The signal generating circuit is reset in response to a speed signal when the speed is in a prescribed range, thus controlling the phase convergence time in response to changes in the speed, i.e., during speed transients.
At present, users of control systems have a choice between microprocessor-based and pulse logic control techniques. The availability of high-power microprocessors makes them suitable for many applications. Nevertheless, the simplicity, low cost and inherent high precision of pulse logic methods make them a preferred choice whenever applicable.